Mechanisms of fracture and inhomogeneous deformation on transverse tensile test of friction-stir-processed AZ31 Mg alloy

Three kinds of AZ31 plates with different initial grain orientations were friction stir processed (FSP). Detailed microstructure and texture evolutions of the FSP plates were obtained. Fracture behavior and non-uniform plastic deformation were focused on. It was found that initial grain orientation had little effect on grain size and texture evolution in stir zone (SZ), but it greatly affected the texture transition at thermal–mechanical affected zone (TMAZ)/SZ interface. Non-uniform plastic deformation was observed in all the three kinds of FSP plates along the tensile direction. The strength difference of the FSP plates was largely reduced compared with the initial plates. Fracture consistently initiated in transition region on advancing side even for the 90° specimen, in which no sudden texture transition was present at TMAZ/SZ interface. As revealed in electron backscatter diffraction (EBSD) maps, SZ-side could be divided into two micro-regions. One was adjacent to TMAZ/SZ interface, named as the region of easy to activate extension twinning (EAET region), where the c-axis was almost parallel with transverse direction (TD). The other was slightly away from TMAZ/SZ interface, named as the region of easy to activate basal slip (EABS region), where the c-axis was tilted about 45° towards TD. The activation of different deformation mechanisms in SZ-side may cause plastic deformation incompatibility and initiate fracture. An “embossed” phenomenon appeared in SZ-center during the transverse tensile tests of all the three kinds of FSP plates, which was related to the distribution of basal plane and caused by the different deformation between SZ-center and SZ-side. The activation of basal slip in EABS region was considered as a major role in the formation of the “embossed phenomenon”. The findings of this study extend understanding the mechanism of fracture and inhomogeneous deformation behaviors of FSP Mg alloys.